1. Field of the Invention
This invention relates to a double buffer system for preventing color reversion of the pH indicator dye which is used to detect bacterial growth in a biological indicator for monitoring the efficacy of a sterilization process.
2. Description of the Related Art
A variety of sterilization systems are used to sterilize medical devices in hospitals and other medical facilities. Steam, heat, ethylene oxide, and hydrogen peroxide are commonly used as sterilants. The STERRAD(copyright) sterilization system, available from Advanced Sterilization Products of Irvine, California, is an exemplary sterilization system which utilizes a combination of hydrogen peroxide and plasma to sterilize medical equipment.
In all of these sterilization processes, biological indicators are commonly used to confirm the effectiveness of the sterilization process. The biological indicators generally include a carrier which has been inoculated with spores or other microorganisms. The effectiveness of the sterilization is assessed by determining whether subjecting the biological indicator to the sterilization cycle destroys all of the microorganisms on the carrier.
The biological indicator is placed into the sterilizer together with the equipment to be sterilized. After the completion of the sterilization process, the biological indicator is removed from the sterilizer, and the carrier is immersed into a sterile culture medium. The culture medium and carrier are incubated for a predetermined time at an appropriate temperature. At the end of the incubation period, an indicator is used to determine whether any microorganisms have survived. If no growth of the microorganisms occurs, it is assumed that the articles in the sterilizer have been properly sterilized. If microorganism growth has occurred, the articles in the sterilizer may not be sterile.
A pH indicating chemical which changes color with a change in pH can be used as an indicator, because acidic byproducts are formed when microorganisms metabolize the growth medium. An acidic change in the pH of the growth medium in the biological indicator therefore indicates bacterial growth. Alternatively, the indicator can be, for example, turbidity in the culture medium which results from cell colonies produced by bacterial growth.
In some biological indicators, the microorganisms, the culture medium, and the indicator are packaged in a way which permits the microorganisms, the culture medium, and the indicator to be combined without exposing the biological indicator to the non-sterile surroundings. This type of biological indicator is a xe2x80x9cself-contained biological indicatorxe2x80x9d or SCBI. Use of SCBIs simplify the test process and minimize the chance that external contamination could affect the test results.
McCormick et al. (U.S. Pat. No. 4,743,537), for example, disclose a SCBI having a compartment with a permeable opening which permits the transmission of sterilant gas or steam while preventing the passage of microorganisms into or out of the compartment. The compartment contains a breakable ampoule with a culture medium. Indicating microorganisms are placed on one end of a wick, and the wick is placed in the compartment, with the end of the wick containing the microorganisms away from the media-containing ampoule and adjacent the permeable opening. After the sterilization process, the device is removed from the sterilizer, and the ampoule is broken, releasing the culture medium into the compartment. The device is incubated and examined to determine whether or not microorganism growth has occurred.
The apparatus and the method of the present invention address the previously unrecognized cause of a problem when using biological indicators containing indicator dyes for detecting microbial growth to determine the efficacy of a sterilization process.
One aspect of the invention involves a method for determining the effectiveness of a disinfection or sterilization process. The method includes providing a carrier with microorganisms on the carrier, exposing the carrier to the disinfection or sterilization process, and incubating the carrier in a growth medium to determine whether the microorganisms grow in the growth medium. The growth medium contains a first buffer with a first pKa, a second buffer with a second pKa, and a pH-sensitive dye. The carrier is incubated in the growth medium after exposure to the disinfection or sterilization process. Growth of the microorganisms in the growth medium generates acid, changing the pH in the growth medium from first pH in a first pH range to a second pH in a second pH range. The first pKa and said second pKa are within the first pH range and the second pH range, respectively.
Advantageously, determining whether the microorganisms have grown involves determining whether the pH changes in the growth medium. Preferably, the pH-sensitive dye has a first color in the first pH range and a second color in the second pH range. The determination of whether the microorganisms have grown includes determining whether the dye changes color from the first color to the second color.
Preferably, there is a lower concentration of buffer having a pKa in the first pH range than buffer having a pKa in the second pH range. Advantageously, the growth medium is contained in an openable enclosure, and the carrier is incubated in the growth medium by opening the enclosure and immersing the carrier in the growth medium. In an embodiment, the carrier and the growth medium are located in a container covered with a gas or vapor permeable but microorganism impermeable barrier. During the disinfection or sterilization process, a germicide gas or vapor diffuses from outside the container into the container through the barrier.
Advantageously, the microorganism is a biological indicating microorganism for the disinfection or sterilization process. Preferably, the disinfection or sterilization process uses stearn, heat, ethylene oxide, hydrogen peroxide, ozone, chlorine dioxide, peracetic acid, performic acid, formaldehyde, glutaraldehyde, ortho-phthalaldehyde, or hypochorite salts as the disinfecting or sterilizing agent.
Another aspect of the invention involves a self-contained biological indicator including a carrier inside a container. The carrier includes viable microorganisms. At least part of the container is transparent, and the container has an opening which is covered with a gas or vapor permeable but microorganism impermeable barrier. At least one openable enclosure is located inside the container. The enclosure contains a culture medium which is capable of supporting growth of the viable microorganisms and a dye which changes color with a change in pH from a first pH range to a second pH range. The enclosure also contains dual buffer system made up of a first buffer having a first pKa and a second buffer having a second pKa. The first pKa and said second pKa are within the first pH range and the second pH range, respectively.
The carrier may be a porous substrate, a non-porous substrate, an absorbent substrate, or a non-absorbent substrate. Advantageously, the gas or vapor permeable but microorganism impermeable barrier is a nonwoven polyolefin. Preferably, the viable microorganism is a biological indicating microorganism for a disinfection or sterilization process. In an embodiment, the openable container is a breakable glass ampoule. Advantageously, the dye is Bromcresol Purple. Preferably, the first buffer includes at least one phosphate salt. In an embodiment, the second buffer includes at least one acetate salt. Preferably, the acetate salt is sodium acetate.
Advantageously, the dual buffer system contains a lower concentration of buffer having a pKa in the first pH range than buffer having a pKa in the second pH range. Preferably, the self-contained biological indicator also has a cap with at least one opening above the barrier, so that gas or vapor can diffuse into said container through the hole and the barrier. In an embodiment, the self-contained biological indicator also has a chemical indicator for indicating exposure of the self-contained biological indicator to a disinfection or sterilization process.
Another aspect of the invention involves a culture medium which is capable of supporting growth of viable microorganisms. The culture medium includes a nutrient broth, a dye which changes color with a change in pH from a first pH range to a second pH range, and a dual buffer system. The dual buffer system contains a first buffer having a first pKa and a second buffer having a second pKa, where the first pKa and the second pKa are within the first pH range and the second pH range, respectively.
Advantageously the dye in the culture medium is Bromcresol Purple. Preferably, the first buffer includes at least one phosphate salt. In an embodiment, the second buffer includes at least one acetate salt. In an exemplary embodiment, the acetate salt is sodium acetate. Advantageously, the dual buffer system includes a lower concentration of buffer having a pKa in the first pH range than buffer having a pKa in the second pH range.